Alumina, also know as corundum, ranks among the hardest, the most refractory, and the most corrosion-resistant materials. Alumina is a hard oxide.
When used as a coating, alumina can prevent oxygen transport. Under strong electrical fields, alumina, by preventing oxygen transport, can prevent oxygen depletion in ceramic materials. However, impurities in the alumina can lead to degradation in other properties, such as resistivity and voltage breakdown.
The use of the same reference symbols in different drawings indicates similar or identical items.